Runs both binaries over 154 cases and compares stdout, stderr and exit
code: encoding at every wrap width, decode round-trips, canonical-tail
rules, padding placement, concatenated streams, every prefix of a known
encoding, usage errors and file operands.
Locates a genuine GNU base64 rather than trusting /usr/bin/base64, since
distributions increasingly ship uutils there and it disagrees with GNU on
most malformed input — running against it fails 51 of the 154 cases, which
doubles as a negative control for the harness itself.
The three known differences are asserted as expected rather than ignored.
